Materials and Methods
Geometry of the metatarsal bone and proximal phalanx was taken from the freely accessible anatomic model of a right foot ( Giovinco, 2013 ). The specimen preparation process in Hypermesh software (Altair Engineering, Troy, MI, USA) is shown in Figure 1 . The STL geometry was scaled in Hypermesh software to obtain the proximal phalanx length consistent with the mean value of 32.7 mm reported in ( Munuera, Polo & Rebollo, 2008 ). Then the bone surfaces in the MTP-1 joint were virtually cut to a congruent spherical shape, simulating the use of a concave proximal reamer and a convex distal reamer during surgery. The dorsal and medial surfaces of bones were also trimmed to facilitate proper fixation of the plate. A part with a rectangular cross-section was added at the “distal” end of the proximal phalanx to enable specimens to be loaded at the centre of pressure under the great toe. The metatarsal bone was cut and a cylindrical part was added to allow the bone to be fixed in a jig as close to the MTP-1 joint as possible. The geometry of the bones prepared in this way was used in the 3D printing of composite joint models.

Artificial models of the metatarsal bone and proximal phalanx were prepared with fused filament fabrication (FFF), using an Ultimaker 3 Extended 3D printer (Ultimaker B.V., Utrecht, Netherlands). In the manufacturing process of bone models, the Ultimaker white polylactide was applied. Due to the irregular shape of bone models, the water-soluble Ultimaker polyvinyl alcohol was used to build the supports. The selected parameters of the manufacturing process are described in Table 1 .

Three techniques were used in the MTP-1 joint arthrodesis (see Figure 2 ): medially positioned locking plate (Medial LP); dorsally positioned locking plate with interfragmentary lag screw (dorsal LP lag screw); medially positioned locking plate with interfragmentary lag screw (medial LP lag screw).

medially positioned locking plate (Medial LP);

dorsally positioned locking plate with interfragmentary lag screw (dorsal LP lag screw);

medially positioned locking plate with interfragmentary lag screw (medial LP lag screw).

The arthrodesis of the seven pairs of bones for each group was performed by an experienced orthopaedic surgeon using the same titanium locking plate (short metatarsophalangeal plate, Medgal Sp. z o.o, Księżno, Poland) and six 2.4 mm locking screws. The right plate was used in the dorsal position and the left plate in the medial position, as it was better suited to the geometry of the proximal phalanx in the medial approach. In the second and third groups, an additional 2.7 mm cortical lag screw was used between the first metatarsal and proximal phalanx. The relative position of bones during arthrodesis was standardized using a dedicated composite jig ( Figure 3 ).

A total of 21 samples were tested in a Zwick/Roell Z10 universal testing machine (ZwickRoell GmbH & Co. KG, Ulm, Germany). The cylindrical end of the first metatarsal and the rectangular end of the proximal phalanx were clamped in custom-made stainless steel jigs, see Figure 4A . Such an experimental setup did not limit the relative displacements between bones and simulated the worst loading conditions with the fixed metatarsal bone and the proximal phalanx loaded perpendicular to the plantar surface of the great toe. An initial load of 1 N was applied in the tests. The MTP-1 joint samples were loaded to failure with a constant speed of 5 mm/min of the upper crosshead to simulate quasi-static loading conditions during the bending test.

To characterize the mechanical behaviour of the MTP-1 joint during the bending test, the digital image correlation (DIC) method was utilized. Images of the lateral surface of the samples covered with a stochastic pattern were captured every 1 s by two cameras with a resolution 4096 × 3000 pixels, being a part of an ARAMIS MC 3D 12M system (GOM GmbH, Braunschweig, Germany). After measurements, the collected photographs were processed by ARAMIS Professional software. Figure 4B shows the view from the left camera at the beginning of the test. To analyse the displacements during the bending test, three control points were selected (P1, P2 and P3, see Figure 4B ).

A construct flexural stiffness was calculated by fitting the least squares regression line to a force-displacement curve over the 5–200 N range. Additionally, an initial flexural stiffness was computed as the initial slope of this curve. The vertical displacement at point P3 was used in the stiffness calculations because point P3 corresponds to the centre of pressure under the great toe. The 200 N load was chosen as an estimate of the maximum force under the great toe which may be 25% of the body weight in the stance phase of gait ( Ledoux & Hillstrom, 2002 ). The relationship between the force (5–200 N range) and vertical relative displacement between points P1 and P2 was used to determine the shear stiffness of the fixation.

Maximum load to the first failure was defined as the extreme load until a force drop greater than 50 N was observed. The horizontal and vertical displacements between points P1 and P2 for two levels of load (50 N and 200 N) were calculated as a measure of the opening displacement and transverse displacement at the joint space, respectively. The force of 50 N corresponds to a partial load of the MTP-1 joint in a healing shoe in the postoperative period.

An analysis of variance (ANOVA) was performed using R version 4.2.1 software ( R Core Team, 2022 ) to examine the effect of the fixation technique on the stiffness of the MTP-1 joint arthrodesis and the maximum load. The homogeneity of variance was verified using Levene’s test (centre = median). Tukey’s Honest Significant Difference (Tukey’s HSD) method with a 95% family-wise confidence level was employed for the pair-wise comparisons between the fixation constructs. The interfragmentary displacements and the shear stiffness were compared between all pairs of fixation techniques using Tamhane’s T2-test for unequal variances.